Devices of the type here contemplated, e.g. light-emitting diodes, semiconductive lasers or Gunn microwave diodes, generally require the flow of a biasing current designed to establish a suitable operating point on a more or less linear part of their characteristic in the absence of an input signal. This operating point is usually located at or somewhat above a predetermined threshold which, however, can vary from one device to another and is also subject to change, due to such factors as temperature fluctuations and aging, in a given device. Thus, the potential established by the biasing current at the driving input of the device is neither exactly predictable nor constant.
Any potential difference existing under zero-signal conditions between this driving input and the output of an associated signal amplifier may be harmful to that amplifier and/or to the device itself besides impairing the response of that device to the applied driving signal.